Make shared atomic values persistent across replica/writer restarts

[kjnilsson]

The atomic values that is shared by osiris processes to allow readers to know the committed offset without having to call into the osiris process is invalidated if the osiris process is restarted. To avoid this we should persist the atomic in an ETS table so that an osiris process that is restarted will re-use the previous atomic.

This ensure readers don't have to re-initialise their log states to resume reading after an osiris process restart.